Communication method and communication apparatus by station that operates in power safe mode in wireless lan system

ABSTRACT

Provided is a communication method by a station (STA) that operates in a power safe mode in a wireless LAN system. The method comprises: transmitting a probe request frame to an access point (AP), wherein the probe request frame includes system information identification information for identifying system information acquired by the STA; and receiving a probe response frame as a response to the probe request frame from the AP. The probe response frame includes a time stamp value related to the point of time of transmitting the probe response frame, and if it is determined, based on the system information identification information, that updated information which is not included in said acquired system information exists, the probe response frame further includes said updated information. Also provided is a communication method by an access point (AP) in a wireless LAN system. The method comprises: receiving a probe request frame from a station (STA), wherein the probe request frame includes information related to a wake-up duration time that is a time period in which the STA operates in an awake state; and transmitting a probe response frame to the STA as a response to the probe request frame. The probe response frame is transmitted within said wake-up duration time.

BACKGROUND OF THE INVENTION

The present invention relates to a wireless communication, and moreparticularly, to a communication method performed by a station (STA)that operates in a power save mode in a Wireless Local Area Network(WLAN) system and to the apparatus for supporting the method.

RELATED ART

With the growth of information communication technology, variouswireless communication technologies have been recently developed. Amongothers, Wireless Local Area Network (WLAN) is a technology that allowsfor wireless access to the Internet at home or business or in a specificservice area using a handheld terminal such as a personal digitalassistant (PDA), a laptop computer, a portable multimedia player (PMP),etc.

IEEE 802.11n is a technology standard that has been recently establishedin order to overcome the limit to communication speed that has beenrecognized as a weakness of WLAN. IEEE 802.11n aims to increase networkspeed and reliability and expand coverage of a wireless network. Morespecifically, the IEEE 802.11n system adopts MIMO (Multiple Inputs andMultiple Outputs) technology that uses multiple antennas at both atransmission unit and a reception unit thereof so as to optimize dataspeed and to minimize transmission errors while supporting a highthroughput (HT) of data processing speed up to 540 Mbps.

In the WLAN system, a station (STA) supports a power save mode. The STAis capable to prevent the unnecessary power consumption by entering andoperating in a doze state. The STA operating in the power save mode iscapable of operating in switching between an awake state and the dozestate. The STA operating in the doze state periodically may enter in theawake state at every specified time or specified interval, and mayidentify whether there is any frame to transmit and/or receive by itselfor not. In addition, the STA may enter in the awake state from the dozestate, and receive information in relation to the system from an accesspoint (AP), thereby maintain the timing synchronization with the AP.

For the STA that operates in the power save mode, the time durationoperating in the doze state may get longer than the previous. The longerthe STA operates in the doze state, the higher possibility of deviationoccurs on the synchronization of the timing and the system informationbetween the STA and the AP. If the time synchronization of the STA isdeviated from the AP, the time maintaining in the awake state may beextended to receive the information relevant to the system from the AP,which may result in the unnecessary power consumption of the STA. Inaddition, if the STA fails to maintain the synchronization of the systeminformation with the AP, it results in the problem that normal WLANservices may not be provided. Therefore, the communication method isrequired in order to maintain the synchronization of the timing and thesystem information between the STA and the AP.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a communication methodperformed by a station (STA) that operates in a power save mode in aWireless Local Area Network (WLAN) system and the apparatus forsupporting the method.

In an aspect, a communication method performed by a station (STA) thatoperates in a power save mode in a wireless LAN system is provided. Themethod includes transmitting a probe request frame to an access point(AP), the probe request frame including a system informationidentification information to identify system information acquired bythe STA, and receiving a probe response frame from the AP in response tothe probe request frame. The probe response frame includes a time stampvalue relevant to a time when the probe response frame beingtransmitted. When it is determined that an updated information isexisted, which is not included in the acquired system information basedon the system information identification information, the probe responseframe further includes the updated information.

The system information identification information may indicate the timestamp value which is in relation to the time when the system informationacquired already is to be received.

It may be determined that the updated information which is not includedin the acquired system information exists, if the time stamp valueindicated by the system information identification information issmaller than the time stamp value at the time when the systeminformation is recently updated by the AP.

The system information identification information may indicate a changesequence that indicates an update version of the acquired systeminformation.

It may be determined that the updated information which is not includedin the acquired system information exists, if the change sequence valueindicated by the system information identification information issmaller than the change sequence value that indicates the update versionof the system information that is managed by the AP.

The probe response frame may further include the change sequence thatindicates an update version of the system information that is managed bythe AP, if the updated information which is not included in the acquiredsystem information is determined to be existed.

In another aspect, a wireless apparatus operable in a wireless LANsystem is provided. The wireless apparatus includes a transceivertransmitting and receiving a radio signal, and a processor functionallyconnected to the transceiver. The processor is configured to transmits aprobe request frame to an access point (AP), wherein the probe requestframe includes a system information identification information thatidentifies system information already acquired by the wirelessapparatus, and receive a probe response frame in response to the proberequest frame. The probe response frame includes a time stamp valuerelevant to a time when the probe response frame is transmitted. Theprobe response frame further includes the updated information, if theupdated information which is not included in the acquired systeminformation is determined to be existed.

In still another aspect, a communication method performed by an accesspoint (AP) in a wireless LAN system is provided. The method includesreceiving a probe request frame from a station (STA), the probe requestframe including information relevant to a wake-up duration time when theSTA operates in an awake state, and transmitting a probe response frameto the STA in response to the probe request frame. The probe responseframe is transmitted within the wake-up duration time.

The probe request frame may include a sleep wait field that indicates atime which the wake-up duration time is started from.

The probe request frame may further include a wake-up duration timefield that indicates the wake-up duration time.

The STA may enter in a doze state at the time of being indicated by thesleep wait field, if the STA receives the probe response frame beforethe duration time of maintaining in the awake state is over, which isindicated by the wake-up duration time field, it enters in the awakestate.

The STA may enter in the doze state when the duration time maintainingin the awake state is over, if the STA fails to receive the proberesponse frame until the duration time maintaining in the awake state isover.

According to the communication method as an exemplary embodiment of thepresent invention, the STA that operates in the power save mode mayactively synchronize the timing by transmitting the probe request frameto the AP. In the environment in which the timing error may be increasedbetween the local clock of the STA and the global clock of the BSS,which caused by elongated time interval operating in the doze state bythe STA, the timing may be synchronized by the request of the STA.Accordingly, the power consumption may be prevented, which may beoccurred when the STA enters in the awake state at the wake-up intervalaccording to the local clock and waits for receiving the beacon frame,and the STA may acquire the system information more quickly because thewaiting time decreases.

According to the communication method as an exemplary embodiment of thepresent invention, the STA that operates in the power save modetransmits the information of identifying the system information to theAP, which is the basis for the system management by itself. The AP mayidentify the presence of the updated information that is not acquired bythe STA among the system information which is the basis of the BSSmanagement at present based on the corresponding system informationidentification information. Consequently, the AP may selectively providethe updated information to the STA, and the STA may acquire the systeminformation more quickly through it, and may carry out the operation inorder to be provided of normal WLAN service.

According to the communication method as an exemplary embodiment of thepresent invention, the STA may be signaling the specific time when theresponse of the AP is given at the request of the timing synchronizationand/or the updated system information. The unnecessary power consumptionof the STA may be prevented, which occurs by maintaining in the awakestate unnecessarily during the delay time due to the preparation of theframe by the AP after entering in the awake state at the wake-upinterval.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating the configuration of a general WirelessLocal Area Network (WLAN) system to which an embodiment of the presentinvention may apply.

FIG. 2 is a view illustrating a physical layer architecture of a WLANsystem supported by IEEE 802.11.

FIG. 3 is a view illustrating an example of power management operation.

FIG. 4 is a flow chart illustrating an example of the communicationmethod performed by the STA that operates in the power save modeaccording to an embodiment of the present invention.

FIG. 5 is a block diagram illustrating an example of the format of thesystem information identification information according to an embodimentof the present invention.

FIG. 6 is a block diagram illustrating another example of the format ofsystem information identification information element according to anembodiment of the present invention.

FIG. 7 is a flow chart illustrating an example of the communicationmethod performed by the STA that operates in a power save mode accordingto another embodiment of the present invention.

FIG. 8 is a block diagram illustrating a wireless apparatus in which theembodiment of the present invention can be implemented.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 is a view illustrating the configuration of a general WirelessLocal Area Network (WLAN) system to which an embodiment of the presentinvention may apply.

Referring to FIG. 1, the WLAN system includes one or more basic servicesets (BSSs). A BSS is a set of stations (STAs) that may be successfullysynchronized with each other and may communicate with each other, and isnot a concept indicating a specific area.

An infrastructure BSS includes one or more non-Access Point (AP)stations (non-AP STA1(21), non-AP STA2(22), non-AP STA3(23), non-APSTA4(24), and non-AP STAa (30)), an AP 10 providing a distributionservice, and a distribution system (DS) linking multiple APs. In theinfrastructure BSS, the AP manages the non-AP STAs of the BSS.

In contrast, an independent BSS (IBSS) is a BSS operating in an ad-hocmode. The IBSS does not include an AP and thus lacks a centralizedmanagement entity. That is, in the IBSS, non-AP STAs are managed in adistributed manner. In the IBSS, all the STAs may be mobile STAs, anddue to no permission to access the DS, constitute a self-containednetwork.

The STA is any functional entity that includes a medium access control(MAC) and a physical layer interface for a radio medium that follow theInstitute of Electrical and Electronics Engineers (IEEE) 802.11standards and in broader concept includes an AP and a non-AP station.

A non-AP STA is a STA that is not an AP, and may also be referred to asa mobile terminal, wireless device, wireless transmit/receive unit(WTRU), user equipment (UE), mobile station (MS), mobile subscriber unitor simply user. Hereinafter, for ease of description, the non-AP STA isdenoted STA.

The AP is a functional entity that provides access to a DS via a radiomedium for a STA associated with an AP. In an infrastructure BSSincluding an AP, communication between STAs is in principle achieved viaan AP, but in case a direct link is set up, the STAs may perform directcommunication between each other. The AP may also be referred to as acentral controller, base station (BS), node-B, BTS (Base TransceiverSystem), site controller, or managing STA.

A plurality of BSSs including the BSS shown in FIG. 1 may be connectedto each other via a distribution system (DS). The plurality of BSSslinked with each other through a DS is referred to as an extendedservice set (ESS). The APs and/or STAs included in the ESS maycommunicate with each other, and in the same ESS, STAs may travel fromone BSS to another BSS while maintaining seamless communication.

In the WLAN system according to IEEE 802.11, the basic access mechanismof Medium Access Control (MAC) is a Carrier Sense Multiple Access withCollision Avoidance (CSMA/CS) mechanism. The CSMA/CS mechanism is alsoreferred to as Distributed Coordination Function (DCF) of IEEE 802.11MAC, and basically, it adopts a “listen before talk” access mechanism.Following such type of access mechanism, an AP and/or STA senses a radiochannel or medium prior to transmission. If as a result of the sensing,the medium is determined to be in idle state, frame transmission isinitiated through the medium. On the contrary, if the medium is sensedto be in occupied state, the AP and/or STA sets a deferred time formedium access and waits without starting its own transmission.

The CSMA/CS mechanism includes virtual carrier sensing in addition tophysical carrier sensing in which an AP and/or STA directly senses amedium. The virtual carrier sensing is to make up for a problem that mayoccur in connection with medium access, such as hidden node problem. Inorder for virtual carrier sensing, the MAC of the WLAN system makes useof a network allocation vector (NAV). The NAV is a value by which an APand/or STA currently using a medium or having authority to use themedium informs other AP and/or STA of a time remaining until the mediumturns available. Accordingly, the value set by the NAV corresponds to aperiod during which the use of the medium is scheduled by the AP and/orSTA transmitting a frame.

The IEEE 802.11 MAC protocol, together with a DCF, offers a HybridCoordination Function (HCF) that is based on a Point CoordinationFunction (PCF) that periodically performs polling so that all receivingAPs and/or STAs may receive data packets in polling-based synchronizedaccess scheme with the DCF. The HCF has an Enhanced Distributed ChannelAccess (EDCA) that has a contention-based access scheme for providingdata packets to multiple users and HCCA (HCF Controlled Channel Access)that uses contention free-based channel access scheme using a pollingmechanism. The HCF includes a medium access mechanism for enhancingQuality of Service (QoS) of WLAN and may transmit QoS data in both acontention period (CP) and contention free period (CFP).

In the WLAN system, the network discovery process is implemented as ascanning procedure. The scanning procedure is separated into passivescanning and active scanning. The passive scanning is achieved based ona beacon frame that is periodically broadcast by an AP. In general, anAP in the WLAN system broadcasts a beacon frame at a specific interval(for example, 100 msec). The beacon frame includes information on a BSSmanaged by it. The STA passively awaits reception of the beacon frame ata specific channel. When obtaining the information on the network byreceiving the beacon frame, the STA terminates the scanning procedure atthe specific channel. The STA need not transmit a separate frame inachieving passive scanning, and the passive scanning is rather done oncethe beacon frame is received. Accordingly, the passive scanning mayreduce the overall overhead. However, it suffers from a scanning timethat is increased in proportion to the transmission period of the beaconframe.

The active scanning is that the STA actively broadcasts a probe requestframe at a specific channel to request that all the APs to receive theprobe request frame send network information to the STA. The Table 1below represents information elements which may be included in the proberequest frame.

TABLE 1 Order Information Element 1 SSID(Service Set ID 2 SupportedRates 3 Request Information 4 Extended Supported Rates 5 DSSS(DirectSequence Spread Spectrum) Parameter Set 6 Supported Operation Classes 7HT(High Throughput) Capabilities 8 20/40 BSS(Basic Service Set)Coexistence 9 Extended Capability Last Vender SpecificWhen receiving the probe request frame, an AP waits for a random time soas to prevent frame collision, and then includes network information ina probe response frame, then transmits the probe response frame to theSTA. The Table 2 below represents information elements which may beincluded in the probe response frame.

TABLE 2 Order Information Element 1 Timestamp 2 Beacon Interval 3Capability 4 SSID(Service Set ID) 5 Supported Rates 6 FH(FrequencyHopping) Parameter Set 7 DSSS Parameter Set 8 CF(Contention Free)Parameter Set 9 IBSS(Independent BSS) Parameter Set 10 Country 11 FHParameters 12 FH pattern Table 13 Power Constraint 14 Channel SwitchAnnouncement 15 Quiet 16 IBSS DFS(Direct Frequency Selection 17TPC(Transmit Power Control) Report 18 ERP(Extended Rate PHY 19 ExtendedSupported Rates 20 RSN(Robust Security Network 21 BSS Load 22EDCA(Enhanced Distributed Channel Access) Parameter Set 23 MeasurementPilot Transmission 24 Multiple BSSID 25 RM(Radio Measurement) EnabledCapabilities 26 AP Channel Report 27 BSS Average Access Delay 28 Antenna29 BSS Available Admission Capacity 30 BSS AC(Access Category) AccessDelay 31 Mobility Domain 32 DSE(Dynamic STA Enablement) RegisteredLocation 33 Extended Channel Switch Announcement 34 Supported OperationClasses 35 HT(High Throughput) Capabilities 36 HT Operation 37 20/40 BSSCoexistence 38 Overlapping BSS Scan Parameters 39 Extended CapabilityLast-1 Vender Specific Last-2 Requested ElementsThe STA receives the probe response frame to thereby obtain the networkinformation, and the scanning procedure is then ended. The activescanning may get scanning done relatively quickly, but may increase theoverall network overhead due to the need of a frame sequence that comesfrom request-response.

When finishing the scanning procedure, the STA selects a network per aspecific standard on itself and then performs an authenticationprocedure alongside the AP. The authentication procedure is achieved in2-way handshake. When completing the authentication procedure, the STAproceeds with an association procedure together with the AP.

The association procedure is performed in two-way handshake. First, theSTA sends an association request frame to the AP. The associationrequest frame includes information on the STA's capabilities. Based onthe information, the AP determines whether to allow association with theSTA. When determining whether to allow association, the AP transmits anassociation response frame to the STA. The association response frameincludes information indicating whether to allow association andinformation indicating the reason for association being allowed orfailing. The association response frame further includes information oncapabilities supportable by the AP. In case association is successfullydone, normal frame exchange is done between the AP and STA. In caseassociation fails, the association procedure is retried based on theinformation on the reason for the failure included in the associationresponse frame or the STA may send a request for association to otherAP.

In order to overcome limit to speed that is considered to be a weaknessin WLAN, IEEE 802.11n has been established relatively in recent years.IEEE 802.11n aims to increase network speed and reliability whileexpanding wireless network coverage. More specifically, IEEE 802.11nsupports high throughput (HT) that reaches data processing speed up to540 Mbps and is based on MIMO (Multiple Inputs and Multiple Outputs)technology that adopts multiple antennas at both transmission end andreception end in order to optimize data speed and minimize transmissionerrors.

As WLAN spreads and more diversified applications using WLAN show up, aneed for a new WLAN system arises for supporting a higher throughputthan the data processing speed supported by IEEE 802.11n. The WLANsystem supporting very high throughput (VHT) is a subsequent version ofthe IEEE 802.11n WLAN system, which is a new one recently suggested tosupport a throughput more than 500 Mbps for a single user and dataprocessing speed more than 1 Gpbs for multiple users in an MAC serviceaccess point (SAP).

Advancing further than the existing WLAN system supporting 20 MHz or 40MHz, the VHT WLAN system intends to support 80 MHz, contiguous 160 MHz,non-contiguous 160 MHz band transmission and/or more bandwidthtransmission. Further, the VHT WLAN system supports 250 QuadratureAmplitude Modulation (QAM) that is more than a maximum of 64QAM of theexisting WLAN system.

Since the VHT WLAN system supports a Multi User-Multiple Input MultipleOutput (MU-MIMO) transmission method for higher throughput, the AP maytransmit a data frame simultaneously to at least one or more MIMO-pairedSTAs. The number of paired STAs may be maximally 4, and when the maximumnumber of spatial streams is eight, each STA may be assigned up to fourspatial streams.

Referring back to FIG. 1, in the WLAN system shown in the figure, the AP10 may simultaneously transmit data to a STA group including at leastone or more STAs among a plurality of STAs 21, 22, 23, 24, and 30associated with the AP 10. In FIG. 1, by way of example, the AP conductsMU-MIMO transmission to the STAs. However, in a WLAN system supportingTunneled Direct Link Setup (TDLS) or Direct Link Setup (DLS) or meshnetwork, a STA to transmit data may send a Physical Layer ConvergenceProcedure (PLCP) Protocol Data Unit (PPDU) to a plurality of STAs usingan MU-MIMO transmission scheme. Hereinafter, an example where an APtransmits a PPDU to a plurality of STAs according to an MU-MIMOtransmission scheme is described.

Data may be transmitted through different spatial streams to each STA.The data packet transmitted by the AP 10 may be referred to as a PPDU,which is generated at the physical layer of the WLAN system andtransmitted, or a frame as a data field included in the PPDU. That is,the PPDU for Single User-Multiple Input Multiple Output (SU-MIMO) and/orMU-MIMO or data field included in the PPDU may be called a MIMO packet.Among them, the PPDU for MUs may be called an MU packet. In the exampleof the present invention, assume that a transmission target STA groupMU-MIMO-paired with the AP 10 includes STA1 21, STA2 22, STA3 23, andSTA4 24. At this time, no spatial stream is assigned to a specific STAin the transmission target STA group, so that no data may be transmittedto the specific STA. Meanwhile, assume that STAa 30 is associated withthe AP but is not included in the transmission target STA group.

In the WLAN system, an identifier may be assigned to the transmissiontarget STA group in order to support MU-MIMO transmission, and thisidentifier is denoted group ID. The AP sends a group ID management frameincluding group definition information for allocating group IDs to theSTAs supporting MU-MIMO transmission and accordingly the group IDs areassigned to the STAs before PPDU transmission. One STA may be assigned aplurality of group IDs.

Table 3 below represents information elements included in the group IDmanagement frame.

TABLE 1 Order Information 1 Category 2 VHT action 3 Membership status 4Spatial stream position

The category field and VHT action field are configured so that the framecorresponds to a management frame and to be able to identify being agroup ID management frame used in a next-generation WLAN systemsupporting MU-MIMO.

As in Table 3, the group definition information includes membershipstatus information indicating whether to belong to a specific group ID,and in case of belonging to the group ID, information indicating thenumber of position to which the spatial stream set of the STAcorresponds in all the spatial streams according to MU-MIMOtransmission.

Since one AP manages a plurality of group IDs, the membership statusinformation provided to one STA needs to indicate whether the STAbelongs to each of the group IDs managed by the AP. Accordingly, themembership status information may be provided in the form of an array ofsubfields indicating whether it belongs to each group ID. The spatialstream position information indicates the position of each group ID, andthus, may be provided in the form of an array of subfields indicatingthe position of a spatial stream set occupied by the STA with respect toeach group ID. Further, the membership status information and spatialstream position information for one group ID may be implemented in onesubfield.

The AP, in case of sending a PPDU to a plurality of STAs through anMU-MIMO transmission scheme, transmits the PPDU, with informationindicating a group identifier (group ID) in the PPDU as controlinformation. When receiving the PPDU, a STA verifies whether it is amember STA of the transmission target STA group by checking the group IDfield. If the STA is a member of the transmission target STA group, theSTA may identify what number of position where the spatial stream settransmitted to the STA is located in the entire spatial stream. The PPDUincludes information on the number of spatial streams allocated to thereceiving STA, and thus, the STA may receive data by discovering thespatial streams assigned thereto.

FIG. 2 is a view illustrating a physical layer architecture of a WLANsystem supported by IEEE 802.11.

The IEEE 802.11 physical (PHY) architecture includes a PHY LayerManagement Entity (PLME), a Physical Layer Convergence Procedure (PLCP)sublayer 210, and a Physical Medium Dependent (PMD) sublayer 200. ThePLME provides a function of managing the physical layer in cooperationwith the MAC Layer Management Entity (MLME). The PLCP sublayer 210delivers an MAC Protocol Data Unit (MPDU) received from the MAC sublayer220 to the PMD sublayer in response to an instruction of the MAC layerbetween the MAC sublayer 220 and the PMD sublayer 200 or delivers aframe coming from the PMD sublayer 200 to the MAC sublayer 220. The PMDsublayer 200 is a PLCP lower layer and enables transmission andreception of a physical layer entity between two stations through aradio medium. The MPDU delivered by the MAC sublayer 220 is denoted aPhysical Service Data Unit (PSDU) in the PLCP sublayer 210. The MPDU issimilar to the PSDU, but in case an aggregated MPDU (A-MPDU) obtained byaggregating a plurality of MPDUs is delivered, each MPDU may bedifferent from each PSDU.

The PLCP sublayer 210 adds an additional field including informationneeded by a physical layer transceiver while delivering a PSDU from theMAC sublayer 220 to the PMD sublayer 200. At the time, the added fieldmay include a PLCP preamble to the PSDU, a PLCP header, or tail bitsnecessary for turning a convolution encoder back into the zero state.The PLCP sublayer 210 receives from the MAC sublayer a TXVECTORparameter including control information necessary to generate andtransmit a PPDU and control information necessary for the STA to receiveand analyze a PPDU. The PLCP sublayer 210 uses information included inthe TXVECTOR parameter in generating a PPDU including the PSDU.

The PLCP preamble plays a role to let the receiver prepare for asynchronization function and antenna diversity before the PSDU istransmitted. The data field may include a coded sequence where the PSDU,padding bits attached to the PSDU, a service field including a bitsequence for initializing a scrambler, and the tail bits are encoded. Atthe time, as an encoding scheme, depending on the encoding schemesupported by the STA receiving the PPDU, Binary Convolution Coding (BCC)encoding or Low Density Parity Check (LDPC) encoding may be selected.The PLCP header includes a field including information on the PPDU to betransmitted.

The PLCP sublayer 210 adds the above-described fields to the PSDU tothereby generate a PPDU and transmits the PPDU to a receiving stationvia the PMD sublayer, and the receiving STA receives the PPDU andobtains the information necessary for restoring data from the PLCPpreamble and PLCP header and restores data. The PLCP sublayer of thereceiving station delivers to the MAC sublayer the RXVECTOR parameterincluding the control information contained in the PLCP header and thePLCP preamble and may analyze the PPDU and obtain data in the receivingstate

Always sensing a channel for frame transmission and reception causes theSTA to continue to consume power. The power consumption in the receptionstate makes little difference as compared with the power consumption inthe transmission state, so that keeping the reception state causes theSTA battery powered to consume relatively more power. Accordingly, whenin the WLAN system a STA conducts channel sensing while continuouslymaintaining the reception waiting state, inefficient power consumptionmay arise without particularly increasing WLAN throughput, and thus, itis inappropriate in view of power management.

To compensate for such problems, the WLAN system supports a powermanagement (PM) mode for a STA. The STA power management mode isseparated into an active mode and a power save (PS) mode. The STAoperates basically in the active mode. The STA operating in the activemode maintains an awake state. That is, the STA remains at a state ofbeing able to perform normal operation such as frame transmission andreception or channel sensing.

When in normal operation, the STA shifts between the doze state andawake state. In the doze state, the STA operating with the minimum powerand does not receive radio signals including data frames from the AP.Further, in the doze state, the STA does not conduct channel sensing.

As the STA operates as long as possible, power consumption decreases, sothat the operation period of the STA is increased. However, since frametransmission and reception is impossible in the doze state, it cannot beleft at the operation state unconditionally. In case there is a frame tobe transmitted from the STA operating in the doze to the AP, the STAshifts to the awake state, thereby able to receive frames. However, incase the AP has a frame to be transmitted to the STA operating in thedoze state, the STA cannot receive the frame nor is the STA able to beaware of the existence of the STA. Accordingly, the STA may require theoperations of being aware of whether there is a frame to be sent to theSTA, and if any, shifting to the awake state at a specific period so asto receive the frame. This is described below in connection with FIG. 3.

FIG. 3 is a view illustrating an example of power management operation.

Referring to FIG. 3, the AP 310 sends a beacon frame to STAs in a BSS ata constant period (S310). The beacon frame includes a Traffic IndicationMap (TIM) information element. The TIM element includes informationindicating that the AP 310 buffers a bufferable frame (or bufferableunit; BU) for the STAs associated with the AP 310 and that the frame isto be sent. The TIM element includes a TIM used to indicate a unicastframe and a Delivery Traffic Indication Map (DTIM) used to indicate amulticast or broadcast frame.

The AP 310 transmits a DTIM once every three beacon frames oftransmission.

STA1 321 and STA2 322 are STAs operating in PS mode. STA1 321 and STA2322 shift from the doze state to the awake state at every wakeupinterval of a specific period so that the STAs may receive the TIMelement transmitted from the AP 310.

A specific wakeup interval may be configured so that STA1 321 may shiftto the awake state at every beacon interval to thus receive a TIMelement. Accordingly, when the AP 310 first sends out a beacon frame(S311), STA1 321 switches to the awake state (S321). STA1 321 receivesthe beacon frame and obtains the TIM element. In case the obtained TIMelement indicates that a bufferable frame to be sent to STA1 321 isbeing buffered, STA1 321 transmits a PS-poll frame to the AP 310 torequest that the AP 310 send a frame (S321 a). In response to thePS-poll frame, the AP 310 sends a frame to STA1 321 (S331). Whencompletely receiving the frame, STA1 321 turns back to the doze state.

When the AP 310 sends out a second beacon frame, since the medium isoccupied, for example, as if another device gains access to the medium,the AP 310 fails to send a beacon frame at exact beacon interval and maydeferred transmission of the beacon frame (S312). In such case, STA1 321turns its operation mode to the awake state according to the beaconinterval, but cannot receive the deferred beacon frame, so that STA1 321switches back to the doze state (S322).

When the AP 310 sends out a third beacon frame, the beacon frame mayinclude a TIM element that is set as DTIM. However, since the medium isoccupied, the AP 310's transmission of the beacon frame is deferred(S313). STA1 321 switches to the awake state in accordance with thebeacon interval and may obtain the DTIM through the beacon frametransmitted by the AP 310. The DTIM obtained by STA1 321 indicates thatthere is no frame to be transmitted to STA1 321 and that there is aframe for other STA. Accordingly, STA1 321 shifts back to the dozestate. The AP 310, after transmission of the beacon frame, sends a frameto the STA (S332).

The AP 310 sends a fourth beacon frame (S314). However, STA1 321 couldnot obtain the information indicating that a bufferable frame for itselfremains buffered through the previous twice reception of the TIMelement, and thus, STA1 321 may adjust the wakeup interval for receptionof a TIM element. Or, in case the beacon frame transmitted by the AP 310includes signaling information for adjusting the wakeup interval valueof STA1 321, the wakeup interval value of STA1 321 may be adjusted. Inthis example, STA1 321 may change its configuration so that shift of theoperation state for receiving a TIM element is performed at every threebeacon intervals rather than at every beacon interval. Accordingly, STA1321 stays at the doze state after the AP 310 sends a fourth beacon frame(S314) and when the AP 310 transmits a fifth beacon frame (S315), andthus, it cannot obtain the TIM element.

When the AP 310 sends out a sixth beacon frame (S316), STA1 321 switchesto the awake state and obtains the TIM element included in the beaconframe (S324). The TIM element is a DTIM indicating that there is abroadcast frame, so that STA1 321 does not transmit a PS-poll frame tothe AP 310 and receives a broadcast frame transmitted by the AP 310(S334).

Meanwhile, the wakeup interval configured in STA2 322 may have a longerperiod than that of STA1 321. Accordingly, when the AP 310 sends a fifthbeacon frame (S315), STA2 322 may switch to the awake state to receive aTIM element (S325). STA2 322 is aware that there is a frame to be sentthereto through the TIM element, and in order to request transmission,sends a PS-poll frame to the AP 310 (S325 a). The AP 310 sends a frameto STA2 322 in response to the PS-poll frame (S333).

In order to operate the power save mode as shown in FIG. 3, the TIMelement includes a TIM indicating whether there is a frame to be sent tothe STA or a DTIM indicating whether there is a broadcast/multicastframe. The DTIM may be embodied by configuring a field of the TIMelement.

In the WLAN system that is operating based on the above power managementmethod, the STA may synchronize the timing based on the beacon frametransmitted from the AP and/or the time stamp information included inthe probe response frame. That is, the STA may synchronize the localclock in relation to the management of the STA with the time in relationto the management of the AP, thus all elements within the BSS that ismanaged by the AP may operate based on the synchronized global clock orBSS clock.

The STA that operates in the power save mode may enter in the doze stateby turning off the power of the transceiver in case the power save modeis unnecessary. The STA may wake up at the time when the beacon frame istransmitted based on the local clock, and may identify whether there isany frame which is going to be forwarded or forward.

The STA may enter in the awake state and receive the beacon frame. TheSTA may prevent the timing error that may occur during the time when theSTA operates in the doze state, and synchronize the local clock with theglobal clock by receiving the beacon frame. In addition, the STA mayidentify whether the system information is updated or not during thetime when the STA operates in the doze state, and may acquire theupdated system information through the beacon frame in case of beingupdated. What the STA acquires the updated system information byreceiving the beacon frame is the mechanism which is necessary tooperate using the configuration parameters which are commonly configuredbetween the STA and the AP.

Meanwhile, as various communication services such as smart grid ande-Health, or ubiquitous services appear, the Machine to Machine (M2M) tosupport such services draws attention. A sensor for sensing temperatureor moisture, a camera, a home appliance such as a TV, or a bulky machineincluding a factory processing machine or a vehicle may be one elementof an M2M system. Elements constituting an M2M system may transmit andreceive data based on WLAN communication. In case devices of an M2Msystem supports WLAN and configure a network, the system is hereinafterreferred to as an M2M WLAN system.

The M2M-supportive WLAN system has the following features.

1) Large number of stations: Unlike an existing network, M2M is on thebasis of an assumption that a large number of STAs exist within a BSS.This is because, sensors, or the like, installed in houses, companies,and the like, are all considered. Thus, a considerably large number ofSTAs may be connected to a single AP.

2) Low traffic load per STA: Since an STA has a traffic pattern ofcollecting and reporting surrounding information, information is notrequired to be frequently sent and an amount of information is small.

3) Uplink-centered communication: M2M has a structure in which a commandis mainly received by downlink, an action is taken, and result data isreported to uplink. Primary data is generally transmitted in uplink, soin a system supporting M2M, uplink is the core.

4) Power management of STA: An M2M terminal largely operates with abattery, so it is difficult for a user to frequently charge it in manycases. Thus, a power management method for minimizing batteryconsumption is required.

5) Automatic recovery function: it is difficult for a user to directlymanipulate a device constituting an M2M system in a particularsituation, so the device is required to have a self-recovery function.

Hereinafter, the WLAN system that supports the M2M having suchcharacteristics will be described as an example in order to describe anembodiment of the present invention. However, the scope of the presentinvention is not limited to the WLAN system that supports the M2M, butrather may be applied throughout the wireless communication systemsincluding general WLAN system.

In accordance with a server/client structure in a general WLAN system, aclient such as STA sends a request for information to a server, and theserver sends information to the STA in response to the request. At thetime, the server that has provided information may be considered amachine that mechanically collects and offers information, and an entitythat has received the information may be a user using the client. Due tosuch structural nature, downlink-oriented communication technology hasbeen mainly developed in the existing WLAN systems.

On the contrary, in the M2M-supportive WLAN system, an opposite of theabove structure applies. In other words, the client, a machine, gathersand provides information, and the user managing the server may requestinformation. That is, in the M2M-supportive WLAN system, the M2M serverissues a command related to ambient environment measurement to M2M STAsand the M2M STAs conduct operation per the command and report thecollected information to the server, in general communication flow.Unlike the previous, the user happens to access the network in the sideof the server, and the communication flow goes in the oppositedirection. These are structural features of the M2M-supportive WLANsystem.

The frequency of the operation actively carried out by the STA thatoperates based on the control of the network relatively decreases incomparison with the operation of the normal condition. When theoperation of the STA is controlled by the network, the STA operates onlyby the response to the control. Otherwise, the STA may be configured tosave the power unnecessarily consumed by entering in the doze state.

According to the characteristics as such, for the STA that operates inthe power save mode, the time duration operating in the doze state mayget longer than the time duration operating in the previous state. Asthe time gets longer in which the STA operates in the doze state, thetiming error may get increased between the local clock and the globalclock of the STA. In this case, the time difference becomes greaterbetween the estimated time of the STA for the beacon frame to beforwarded based on the local clock and the time for the beacon frameactually being transmitted by the AP. The STA may keep in the awakestate for a long time to receive the beacon frame as much as the timedifference becomes greater, which results in the problem that theunnecessary power consumption may be increased by the STA. Accordingly,a communication method is required in order to decrease the unnecessarypower consumption which is cause by the STA that operates in the powersave mode and to maintain the synchronization of the timing and thesystem information between the STA and the AP.

In order to solve the problem above, the present invention suggests themethod of transmitting the probe request frame to the AP instead of theSTA entering in the awake state and waiting to receive the beacon frame.

FIG. 4 is a flow chart illustrating an example of the communicationmethod performed by the STA that operates in the power save modeaccording to an embodiment of the present invention.

Referring to FIG. 4, the STA that is operating in the doze state entersin the awake state at the specific time by the wake up interval (S410).

The STA that enters in the awake state transmits the probe request frameto the AP after acquiring the channel access authority based on thechannel access mechanism such as the DCF in order to access the channel(S420). The STA may request the updated system information and theinformation relevant to the global clock while operating in the dozestate, by transmitting the probe request frame to the AP.

While the STA broadcasts the probe request frame during the activescanning process, the probe request frame that calls for the updatedsystem information and/or the information relevant to the global clockmay be transmitted to the AP by the unicast method.

The probe request frame may have the format by the above representedtable 1, may further include the system information identificationinformation. The system information identification information is theinformation which is able to identify the system information that theSTA has recently received by the AP. The system informationidentification information may be included in the probe request frame asa format of an element of the system information identificationinformation.

The AP receives the probe request frame that is transmitted from theSTA, and determines whether there exists any information updated whichdoes not belong to the STA among the present system information based onthe system information identification information (S430). Theinformation updated, for example, means an alteration or are-configuration of an information element consisting of the systeminformation like the AP re-configures the values of the EDCA parameterset.

The AP transmits the probe response frame to the STA in response to theprobe request frame (S440). The format of the probe response frametransmitted by the AP may have the format as represented by the table 2.However, if the AP determines that there is any information updated, theAP may transmit the corresponding information updated to the STA withthe information being included in the probe response frame.

The STA receives the probe response frame from the AP, and enters in thedoze state (S450). The STA may update its system information based onthe updated information acquired. In addition, the STA may synchronizethe local clock with the global clock based on the value of a timingsynchronization function (TSF) of the time stamp field included in theprobe response frame.

The process of determining the presence of the updated information thatis not included in the STA among the present system information isimplemented differently according to the way of the specific embodimentof the system information identification information. Hereinafter, anembodiment of the present invention will be described in more detailaccording to an implementation of the system information identificationinformation that may be included in the probe request frame.

1. The case of implementation of the time stamp by using the systeminformation identification information

The time stamp may be implemented by using the system informationidentification information, which is a basis of the AP to identifywhether there is any information updated or not. When updating a part orthe whole of the system information, the AP may save the TSF at thecorresponding time. The value of the TSF will be denoted by Tu below.

If the STA may acquire the system information through receiving thebeacon frame and/or the probe response frame from the AP, the STA maysave the information which is relevant to the beacon frame and/or theprobe response frame, which are/is the most recently received. Thebeacon frame and the probe response frame may include the time stampfield which is containing the information relevant to the time of thecorresponding frame to be transmitted. Accordingly, the terminal maysave the TSF value of the time stamp field which is included in thebeacon frame and/or the probe response frame which has/have beenreceived in the most recently.

When the STA transmits the probe request frame to the AP, the STA mayset up the TSF value as the system information identificationinformation. In this case, the system information identificationinformation may have the format as illustrated by FIG. 5 below.

FIG. 5 is a block diagram illustrating an example of the format of thesystem information identification information according to an embodimentof the present invention.

Referring to FIG. 5, the system information identification informationelement 500 may include an element ID field 510, a length field 520 anda time stamp field 530 that is received.

The element ID field 510 may indicate that the corresponding informationelement is system information identification information.

The time stamp field 530 may be set up as TSF value which is indicatedby the time stamp field including the beacon frame or the probe responseframe which is received in the most recently.

In step, S430, the AP may receive the probe request frame, and maydetermine the presence of the updated information by comparing the Tuvalue and the value of the time stamp field received that is included inthe system information identification information element. The methodfor determination will be described below.

1) In case that the value indicated by the time stamp field received isgreater than or equals to the Tu value

The AP may determine that any updated information is not existed.Accordingly, the AP may transmit the probe response frame in which thetime stamp frame may be included, but the information relevant to thesystem information may not be included.

2) In case that the value indicated by the time stamp field received issmaller than the Tu value

The AP may determine that the updated information is existed.Accordingly, the AP may transmit the prove response frame in which thetime stamp filed and the updated information may be included.

Meanwhile, when more than one information element among the systeminformation is updated, it may be implemented to save and manage each ofTi as a TSF value that is relevant to the updated information, and IEias the updated information corresponding to the TSF value. The updatedinformation may be saved as vector form of [Ti, IEi] by the maximumnumber N or during the maximum time interval of DMax (where, N is aninteger of 1 or greater, DMax is a time interval longer than the dozetime, i is an integer that is greater than or equals to 1 and smallerthan or equals to N, and i increase by 1 as the update timing isdelayed).

In step, S430, the AP may receive the probe request frame, and maycompare the value indicated by the time stamp field which is included inthe probe request frame received with each of the Ti values. By thecomparison, the updated information may be determined as follows.

1) When the value indicated by the time stamp field received is smallerthan a specific Tn, the probe response frame may include the updatedinformation IEi that corresponds to the Ti that is greater than Tn.

2) When the value indicated by the time stamp field received is greaterthan or equals to a specific Tn, the probe response frame may includethe time stamp field, but may not include the updated frame.

2. The case of implementation of a change sequence by using the systeminformation identifying information

As the system information identification information on which is a basisof the AP identifying the presence of the updated information, a changesequence that indicates the version of the system information may beused. When the AP updates a part or the whole of the system information,the AP may update the change sequence, for example, may increase thechange sequence of the specific value. The specific value of the changesequence may be implemented by an integer, and the increment isimplemented by ‘1’. That is, the change sequence may identify the systeminformation of a specific time, and the order of the system informationthat differs each other may be determined by comparing the changesequences.

When the AP transmits the beacon frame and/or the probe response frame,it may transmit including the change sequence that corresponds to thesystem information included. When the STA acquires the systeminformation through receiving the beacon frame and/or the probe responseframe from the AP in the previous time, the STA may save the changesequence which is in the most recently received.

When the STA transmits the probe request frame to the AP, the STA mayset up the change sequence value as the system informationidentification information. In this case, the system informationidentification information may have the format illustrated in FIG. 6below.

FIG. 6 is a block diagram illustrating another example of the format ofsystem information identification information element according to anembodiment of the present invention.

Referring to FIG. 6, the system information identification informationelement 600 may include an element ID field 610, a length field 620 anda change sequence field 630.

The element ID field 610 may indicate that the corresponding informationelement is the system information identification information. The lengthfield 620 may indicate the length of the change sequence field 630.

The change sequence field 630 may set up as the change sequence valuethat is included in the most recent beacon frame or the probe responseframe received by the STA.

In step, S430, the AP may determine the presence of the updatedinformation by comparing the change sequence value indicated by thechange sequence field of the probe request frame with the changesequence value presently set up by the AP. The detailed method fordetermination is described as follows.

1) When the value indicated by the change sequence field is greater thanor equals to the change sequence value that is set up by the AP

The AP may determine that the updated information is not existed.Accordingly, the AP may transmit the probe response frame in which thetime stamp field may be included, but the information relevant to thesystem information may not be included.

2) When the value indicated by the change sequence field is smaller thanthe change sequence value that is set up by the AP

The AP may determine that the updated information is existed.Accordingly, the AP may transmit the probe response frame in which thetime stamp field, the updated information and the change sequencesignaling field that indicates the change sequence that is set up by theAP may be included.

In the embodiment described by FIG. 4, the AP transmits the proveresponse frame right upon receiving the probe request frame from theSTA. However, there may be a case that the AP is unable to response withthe probe response frame without delay when the AP receives the proberequest frame. This is because the time to patch the frame is requiredto transmit the probe response frame after the AP receives the proberequest frame. In addition, since the probe response frame istransmitted by a contention based on the channel access mechanism suchas the DCF, the time of maintaining in the awake state may be elongatedin order for the STA to receive the probe response frame. That mayresult in the problem that the STA consumes the power unnecessarily.Accordingly, when the STA transmits the probe request frame and requeststhe information relevant to the system information of the AP and theglobal clock, the STA may transmit the probe request frame whichincludes the information relevant to the timing when you want to receivethe answer in response to the request.

FIG. 7 is a flow chart illustrating an example of the communicationmethod performed by the STA that operates in a power save mode accordingto another embodiment of the present invention.

Referring to FIG. 7, the STA that operates in the doze state enters inthe awake state at a specific timing by the wake up interval (S710).

The STA that enters in the awake state transmits the probe request frameto the AP after acquiring the channel access authority based on thechannel access mechanism such as the DCF in order to access the channel(S720). The STA may request the changed system information of the AP andthe information relevant to the global clock while the STA operates inthe doze state, by transmitting the probe request frame to the AP.

Meanwhile the STA broadcasts the probe request frame during the activescanning process, the probe request frame that request the updatedsystem information and/or the information relevant to the global clockmay be transmitted to the AP by the unicast method.

The probe request frame may have the format by the above representedtable 1, may further include the system information identificationinformation element including the format and the information that isdescribed above by referring FIGS. 4 to 6.

Meanwhile, the probe request frame may further include the proberesponse time information element. The probe response time informationelement is to indicate the target probe response time, which is the timewhen the AP requests to transmit the probe response frame. Indicatingthe target probe response time may be implemented by indicating thesleep waiting time which is maintaining the sleep mode from the timewhen the probe request frame is transmitted. In this case, the proberesponse time information element may include the sleep waiting field,which may indicate the sleep waiting time.

The STA may enter in the doze state after transmitting the probe requestframe (S730).

The AP receives the probe request frame transmitted from the STA, anddetermines whether there is any updated information which does notbelong to the STA among the present system information, based on thesystem information identification information (S740). What the APdetermines whether any updated information is existed or not isdescribed with reference to FIGS. 4 to 6.

The STA transmits the probe request frame to the AP, and then mayoperate in the doze state till the target probe response time. That is,the STA transmits the probe request frame to the AP, and may notifywhether the AP receive the probe response frame through the proberesponse time information element by switching to the awake state at acertain point.

The AP receives the probe request frame, and may identify the targetprobe response time through the sleep wait field of the probe responsetime information element. The AP may reserve the target probe responsetime as the time for transmitting the probe response frame.

Some other STAs and APs that overhear the probe request frametransmitted from the STA may identify the value indicated by the proberesponse time information element, and may identify the target proberesponse time when the probe response frame is about to be transmittedby the AP which is to answer in response to the relevant probe requestframe. Other APs and STAs may be implemented not to access the channelat the target probe response time in order to guarantee the transmissionof the probe response frame of the AP.

When the sleep wait time is ended, which is indicated by the sleep waitfield of the probe response time information element is over, the STAenters in the awake state (S750).

The AP transmits the probe response frame to the STA (S760). The AP maytry to access the channel in order to transmit the probe response framefrom the target probe response time. When other APs and STAs areimplemented not to access the channel at the target probe response timeby overhearing the probe request frame, the AP may normally access thechannel and transmit the probe response frame at the target proberesponse time. The probe response frame that is transmitted by the APmay be implemented as described above in reference to FIGS. 4 to 6.

The STA that receives the probe response frame from the AP may enter inthe doze state (S760). The STA may update the system information whichbelongs to itself based on the updated information acquired. Inaddition, the STA may synchronize the local clock with the global clockbased on the value of the timing synchronization function (TSF) of thetime stamp field included in the probe response frame.

Meanwhile, if there is no limit on the channel access by other APs andSTAs as such, the AP may not transmit the probe response frame byaccessing the channel at target probe response time, therefore, thetransmission of the probe response frame may be delayed. That may resultin the problem that the STA which enters in the awake state may keepconsuming the power by continuously maintaining in the awake state. Tosolve such a problem, when the STA transmits the probe request frame,the STA may transmit the wake-up duration field with it being includedin the probe response time information element, and which indicates thetime interval operating in the awake state from the target proberesponse time. That is, the probe request frame includes the proberesponse time information element that contains the information relevantto the time interval operating in the awake state in order for the STAto receive the probe response frame from the AP. The probe response timeinformation element may include the sleep wait field and the wake-upduration field. The STA maintains in the awake state during the timeinterval of the wake-up duration time interval indicated by the wake-upduration field from the target probe response time. However, if the STAfails to receive the probe response frame, the STA may enter in the dozestate again when the wake up duration time is over.

The AP may be aware of the time interval in which the STA maintains theawake state through the probe response time information element.Accordingly, the AP may transmit the probe response frame before thewake-up duration time interval is ended, which is indicated by thewake-up duration time field from the target probe response time. If thewake-up duration time interval is ended while the AP is transmitting theprobe response frame to the STA, the AP may interrupt the transmissionof the probe response frame.

According to the communication method as an exemplary embodiment of thepresent invention, the STA that operates in the power save mode mayactively synchronize the timing by transmitting the probe request frameto the AP. In the environment in which the timing error may be increasedbetween the local clock of the STA and the global clock of the BSS,which caused by elongated time interval operating in the doze state bythe STA, the timing may be synchronized by the request of the STA.Accordingly, the power consumption may be prevented, which may beoccurred when the STA enters in the awake state at the wake-up intervalaccording to the local clock and waits for receiving the beacon frame,and the STA may acquire the system information more quickly because thewaiting time decreases.

According to the communication method as an exemplary embodiment of thepresent invention, the STA that operates in the power save modetransmits the information of identifying the system information to theAP, which is the basis for the system management by itself. The AP mayidentify the presence of the updated information that is not acquired bythe STA among the system information which is the basis of the BSSmanagement at present based on the corresponding system informationidentification information. Consequently, the AP may selectively providethe updated information to the STA, and the STA may acquire the systeminformation more quickly through it, and may carry out the operation inorder to be provided of normal wireless LAN service.

According to the communication method as an exemplary embodiment of thepresent invention, the STA may be signaling the specific time when theresponse of the AP is given at the request of the timing synchronizationand/or the updated system information. The unnecessary power consumptionof the STA may be prevented, which occurs by maintaining in the awakestate unnecessarily during the delay time due to the preparation of theframe by the AP after entering in the awake state at the wake-upinterval.

FIG. 8 is a block diagram illustrating a wireless apparatus in which theembodiment of the present invention can be implemented.

Referring to FIG. 8, a wireless apparatus 800 includes a processor 810,a memory 820 and a transceiver 830. The transceiver 830 transmits and/orreceives a wireless signal, which is implemented by a physical layer ofIEEE 802.11 standard. The processor 810 may be functionally connected tothe transceiver 830, and may be set up to determine whether anybufferable frame for itself is buffered or not, which is performed byreceiving the TIM element. The processor 810 may be set up to transmitthe PS-poll frame or SP poll frame of the copied format. The processor810 may be configured to transmit and receive the buffered frame. Theprocessor 810 may be set up to switch between the doze state and/or theawake state according to the transmission and the reception of the TIMelement and the buffered frame. The processor 810 may be configured toimplement the description in reference to FIGS. 4 to 7 according to anembodiment of the present invention.

The processor 810 and/or the transceiver 830 may include an applicationspecific integrated circuit (ASIC), other chipsets, a logic circuitand/or a data processing device. When an embodiment is implemented bysoftware, the above-described technique may be implemented by themodules (a processing, a function, and the like) that perform theaforementioned functions. The modules are stored in the memory 820, andmay be executed by the processor 810. The memory 820 may be included inthe processor 810, and may be functionally connected with the processor810 by various known means with separately positioned outside.

Although the methods are described based on the flow charts as a seriesof steps or blocks in the aforementioned exemplary system, the presentinvention is not limited to the order of the steps. A certain step maytake place differently from the aforementioned steps, with differentorder or at the same time. In addition, it may be understood to one ofordinary skill in the art that the steps shown in the flow chart are notexclusive, but rather may include other steps, and one or more of thesteps may be deleted without influencing the scope of the presentinvention.

1-12. (canceled)
 13. A communication method performed by a station (STA)that operates in a wireless LAN system, the method comprising:receiving, by the STA, first change information; transmitting, by theSTA, a probe request frame including the first change information; andin response to the probe request frame, receiving, by the STA, a proberesponse frame including updated information to be updated by the STA,wherein the updated information is received by the STA after the firstchange information is compared with second change information, whereinthe first change information includes information related to a change ofat least one information element of a beacon frame from previouslyconfigured information elements of the beacon frame, and wherein thesecond change information includes information related to another changeof at least one information element of the beacon frame from previouslyconfigured information elements of the beacon frame updated by thechange of at least one information element.
 14. The method of claim 13,wherein a value of the second change information is larger than a valueof the first change information, and wherein the probe response framefurther includes the second change information.
 15. The method of claim13, wherein the first change information is received via the beaconframe.
 16. The method of claim 13, wherein the probe response framefurther includes a timing synchronization function (TSF) field fortiming estimation.
 17. A wireless apparatus operable in a wireless LANsystem, the wireless apparatus comprising: a transceiver transmittingand receiving a radio signal; and a processor functionally connected tothe transceiver and configured to: receive first change information;transmit a probe request frame including the first change information;and in response to the probe request frame, receive a probe responseframe including updated information to be updated by the STA, whereinthe updated information is received by the STA after the first changeinformation is compared with second change information, wherein thefirst change information includes information related to a change of atleast one information element of a beacon frame from previouslyconfigured information elements of the beacon frame, and wherein thesecond change information includes information related to another changeof at least one information element of the beacon frame from previouslyconfigured information elements of the beacon frame updated by thechange of at least one information element.
 18. The wireless apparatusof claim 17, wherein a value of the second change information is largerthan a value of the first change information, and wherein the proberesponse frame further includes the second change information.
 19. Thewireless apparatus of claim 17, wherein the first change information isreceived via the beacon frame.
 20. The wireless apparatus of claim 17,wherein the probe response frame further includes a timingsynchronization function (TSF) field for timing estimation.
 21. Anaccess point (AP) in a wireless LAN system, comprising: a transceivertransmitting and receiving a radio signal; and a processor functionallyconnected to the transceiver and configured to: transmit first changeinformation; receive a probe request frame including first changeinformation; determine whether to include updated information in a proberesponse frame by comparing between the first change information andsecond change information; and transmit the probe response frameincluding the updated information, wherein the first change informationincludes information related to a change of at least one informationelement of a beacon frame from previously configured informationelements of the beacon frame, and wherein the second change informationincludes information related to another change of at least oneinformation element of the beacon frame from previously configuredinformation elements of the beacon frame updated by the change of atleast one information element.
 22. The AP of claim 21, wherein a valueof the second change information is larger than a value of the firstchange information, and wherein the probe response frame furtherincludes the second change information.
 23. The AP of claim 21, whereinthe first change information is received via the beacon frame.
 24. TheAP claim 21, wherein the probe response frame further includes a timingsynchronization function (TSF) field for timing estimation.